The present disclosure relates to an oil-injected compressor and in particular to an oil-injected screw-type compressor with a motor-powered compressor unit for production of compressed air. The compressor interacts with an oil circuit for lubrication, which is followed by an oil separator device for separation of the oil from the compressed air. A self-resetting temperature switch which switches off the compressor unit when the air/oil mixture flowing in reaches a maximum temperature limit, is provided in the area of the inlet of the compressed air containing oil into the oil separator device.
In addition to oil-injected screw-type compressors, the present disclosure is also used for other types of oil-injected compressors, for example, vane-cell compressors as well. In the case of compressors of the type that is of interest here, oil is injected by an oil circuit into the area of the moving compressor components, and on their bearing points. This lubricates the roller bearings, which are provided in this case and rotate at high speed, and prevents unacceptable heating in the area of the moving compressor components, as a result of friction. Furthermore, the oil is also used to seal the air side from other areas of the compressor. The use of oil-injected compressors such as these extends not only to stationary compressed-air supply installations, but also to mobile applications such as rail vehicle construction, or else to commercial vehicle construction where compressors are used to produce compressed air for the vehicle compressed-air supply system.
Oil-injected compressors, such as oil-injected screw-type compressors, are known from the general prior art. An oil-injected screw-type compressor essentially comprises a compressor unit with at least one pair of compressor screws in the form of rollers, rotating in opposite directions to one another, and intermeshing with one another. This compressor screw arrangement produces compressed air. Air, that is sucked in from the atmosphere from one side, is converted by continuous compression to compressed air, and leaves the compressor unit via a spring-reset outlet valve. The compressor screw arrangement is driven via a drive shaft which extends out of the compressor unit via a seal. A motor, which is flange-connected to it the drive shaft, may be an electric motor. In order to lubricate, seal and cool the compressor unit, it is equipped with an oil circuit starting from an oil sump, supplies oil to the central area of the compressor screw arrangement as well as to roller bearings which are arranged in the area of the end face of the compressor screw arrangement. The oil which is injected here leaves this active area in the direction of an oil sump, which represents the reservoir for the oil circuit. The oil sump is generally located within an oil separator device which follows the oil circuit. The oil separator device is necessary in order to remove the oil from the compressed air again, so that compressed air which is free of oil is available on the output side. Conventionally, the oil separator device is formed essentially from an oil separator which operates in a manner known per se, on the force of gravity principle. The oil, which is separated from the compressed air which contains oil and rises in the oil separator, is gathered in the oil sump. The compressed air which is already partially free of oil and rises in the oil separator is then generally supplied to a cartridge-like fine separator and then leaves the oil separator device via a pressure-maintenance valve arranged on the output side.
According to EN Standard 1012-1, it is not permissible for safe operation of an oil-injected compressor for the oil temperature to exceed 120° C. adjacent to the area in which the compressed air containing oil enters the oil separator device. A temperature switch is normally arranged in this area in order to comply with this Standard. The temperature switch switches on won reaching a temperature of 120° C. and stops the drive for the compressor unit, by switching off the motor. When the temperature falls into the range below 120° C. again, the drive for the compressor unit is enabled again.
U.S. Pat. No. 5,118,260 discloses a temperature switch for a screw-type compressor although, in this case, this is not in the form of an oil-injected screw-type compressor. The temperature switch is arranged at the outlet of the screw-type compressor within an outlet chamber. The compressed air which has been heated by the screw-type compressor flows past the temperature switch. The temperature switch contains an electrical bimetallic element which interrupts the drive to the screw-type compressor when the temperature of the compressed air that has been produced reaches a specific maximum value. In addition to this temperature switch, which is arranged in the area of the compressed air flowing out of the compressor unit, a further temperature switch is located in the area of the electric motor that drives the screw-type compressor, and protects the entire unit against motor overheating.
If a motor-powered compressor unit such as this is provided with oil injection, so that it is necessary to provide a downstream oil separator device in order to separate the oil from the compressed air, this results in the problem. Internal fires or detonations can occur sporadically, despite the measure explained above to prevent overheating within the oil separator device. A singular event such as this normally occurs downstream from the temperature switch, as required in accordance with the Standard cited initially, within the oil separator device. So far, the reason for such an internal fire or detonation has not clearly been explained. In specialist circles it is assumed that this event is the consequence of electrostatic discharges within the oil separator device, producing electrical sparks. Lack of servicing and, in particular, lack of oil can also be considered as detonation causes. A fire or a detonation results in temperatures in the oil separator device and downstream from it which are many times higher than the specified temperature limit of 120° C. Since the temperature on the inlet side of the oil separator device, in particular because of the physical proximity of the oil sump, is matched only slowly to the hot temperature level, the required temperature switch in the area of the compressed air flowing in cannot react sufficiently quickly to the event of a fire or detonation within the oil separator device, or downstream from it. A fire or detonation can result in the components of the oil separator device, which are occasionally also produced from aluminum, burning through. Furthermore, the bearing for the compressor screws can seize as a consequence of overheating or lack of lubrication. In the case of the gray-iron and cast-steel housings normally used, this can even lead to explosive destruction of the compressor. Furthermore, combustion residues also enter the exhaust air. In summary, this singular event can result in hazards to personnel and damage to the compressor as well as consequential damage, which therefore cannot be prevented, or at least limited by the temperature switch required by the Standard.